This invention relates to cable bolts used as ground support in mining and similar operations.
Cable bolt support is progressively gaining a higher proportion of the ground support market in Canada and elsewhere. This is principally due to (1) a cost/length which is comparable to rebar, (2) a 50-75% increase in weight-to-strength ratio compared to rebar, (3) high capacity (25 tonnes) compared to 150 kN for standard rebar and 10 tonnes for a mechanical rock bolt, (4) the versatility of cable bolts due to the fact that relatively long lengths can be coiled for installation from restricted access, and (5) the fact that high bond strength, comparable to rebar, can be developed using modified geometries such as the xe2x80x9cbulgexe2x80x9d cable and the xe2x80x9cGarford bulbxe2x80x9d cable.
The majority of cable bolts are grouted with a Type 10 Portland cement grout which does not attain full strength until 28 days. This has prevented the use of cable bolts as primary support at an advancing face. However, in U.S. coal mines, short lengths of cable are spun in a two-component epoxy-resin grout for primary roadway support. Also, application of epoxy grouted cables has been recently developed for specific hard-rock mining applications, such as ore-pass support. Ground Control Limited of Sudbury Canada is currently marketing a resin grouted cable bolt system called the Strandloc(trademark) bolt. For this system, a threaded fixture is swaged onto the cable to facilitate mixing of the epoxy using a drill. However, tests indicate that this fixture will pull off the cable at 2-3 tonnes, and hence will do little more than hold the screen in place. As a result, to mobilize the full capacity of the cable (25 tonnes) the bolt must be full-column grouted, as opposed to toe-grouted, which becomes expensive for longer lengths.
It is an object of the present invention to obviate the above mentioned disadvantage and to provide a yielding fixture capable of mobilizing substantially full bolt capacity, even with toe-grouted cable bolts.
Another object of the present invention is to produce a yielding fixture attachment to the cable bolt which will have a predetermined pull-off load that may be either constant or variable.
Other objects and advantages of the present invention will be apparent from the following description thereof.
In essence, the present invention concerns a cable bolt having a cable and a yielding fixture at one of its ends, said yielding fixture being tubular and being attached to the cable by providing an insert of a predetermined size within a portion of the cable at a predetermined distance from an end of the cable, thereby forming a bulge in said portion of the cable, and forcing the portion of the cable with the bulge into the tubular yielding fixture so as to achieve a predetermined pull-off load of the yielding fixture. The tubular yielding fixture may be provided with means that will cooperate with a drill or other suitable device to permit spinning of the cable into a grouted hole in the ground which is being supported by said cable. The bulge near the end of the cable can be produced by inserting a suitable element within the structure of the cable by unwinding the end of the cable, placing the element within the structure of the cable and then rewinding the wires of the cable over the element, thereby producing the bulge. The preferred type of element is a slug that can be placed, for example, over the kingwire of the cable with the peripheral wires then being wrapped around the slug, thereby forming the bulge.
According to a preferred embodiment, the present invention involves a cable bolt having a 7-strand cable made of six peripheral wires wound around a middle wire or kingwire and having at one of its ends a tubular yielding fixture attached thereto so as to achieve a predetermined pull-off load. Such tubular yielding fixture may, for example, be a metal tube with a threaded portion at its end and including a nut that can be screwed onto the threaded end of the fixture. In lieu of the threaded portion, the metal tube may be provided at its end with means that would cooperate with a drill or similar device that would be used to rotate the tubular fixture so that the cable attached thereto can be spun when it is inserted into the grouted hole in the rock or ground being supported. Such means at the end of the tube could be a nut welded to the end of the tube or a flattened machined portion that would fit into the head cavity of a drill or otherwise used to spin the cable within the grouted hole. The attachment of the tube to the cable is preferably done in accordance with the present invention by unwrapping the wires at one end of the cable, placing a slug over the kingwire at a predetermined distance from the end of the cable, and then rewrapping the peripheral wires around the slug and the kingwire, thereby forming an enlarged diameter portion or bulge in the cable, and then forcing the end of the cable with the bulge into the tube so as to compress the slug within the bulge and thereby obtain a yielding fixture with a predetermined pull-off load based on the length and thickness of the slug employed. Different sizes, i.e. lengths and thicknesses, of slugs give different pull-off loads that can be measured, for instance with a load cell or similar device used during the insertion of the bulged cable into the tube. Also, such cable bolts can be instrumented with stress measuring rock support devices such as disclosed, for instance, in U.S. Pat. No. 5,929,341, to provide measurement of stress or strain in the ground being supported.
In accordance with the present invention, the method of forming a cable bolt with a tubular yielding fixture attached to an end of a cable, comprises providing an insert of a predetermined size within a portion of the cables at a predetermined distance from the end of the cable, so as to form a bulge in said portion of the cable, and forcing the portion of the cable with the bulge into the tubular fixture so as to achieve a predetermined pull-off load of the yielding fixture.
According to a more specific and preferred embodiment, the method comprises:
(a) spinning-open one end of a 7-strand cable by twisting the peripheral wires in a direction opposite to their inherent helical wrap and unwinding the peripheral wires until a portion of the kingwire is revealed and is followed by a small protuberance formed by the peripheral wires expanding over the kingwire;
(b) placing a slug, e.g. a slug made of mild steel, having a predetermined size and thickness, over the kingwire and sliding it so that it penetrates into the protuberance;
(c) winding the peripheral wires back to their original state with the slug remaining within the protuberance where it becomes enclosed, thereby making the cable diameter to be slightly larger in that portion of the cable, and forming a bulge near the end of the cable;
(d) sliding over the cable a tubular yielding fixture through its sleeve, (e.g. a steel tube with a threaded portion or with a welded nut or the like at its outer end) from the opposite end of the cable, so that the outer end of the fixture (with the threaded portion or the welded nut or the like) is up-front, and moving it until the outer end of the fixture comes into contact with the bulge in the cable where the slug is enclosed, over which it can no longer slide;
(e) forcing the tubular yielding fixture over the bulge in the cable, for example by clamping the fixture and pulling the portion with the bulge into the sleeve of the fixture, while measuring the load required to pull the portion of the cable with the bulge fully into the sleeve, which becomes the pull-off load of the yielding fixture; and, if necessary,
(f) cutting off any part of the cable protruding from the outer end of the yielding fixture.